Ethylene polymers have been used generally and widely as resin materials for various molded articles and are required of different properties depending on the molding method and purpose. For example, polymers having relatively low molecular weights and narrow molecular weight distributions are suitable for articles molded by an injection molding method. On the other hand, polymers having relatively high molecular weights and broad molecular weight distributions are suitable for articles molded by blow molding or inflation molding. In many applications, medium-to-high molecular weight polyethylenes are desirable. Such polyethylenes have sufficient strength for applications which call for such strength (e.g., pipe applications), and simultaneously possess good processability characteristics.
Ethylene polymers having broad molecular weight distributions can be obtained by use of a chromium catalyst obtained by calcining a chromium compound carried on an inorganic oxide carrier in a non-reducing atmosphere to activate it such that at least a portion of the carried chromium atoms is converted to hexavalent chromium atoms (Cr+6) commonly referred to in the art as the Phillips catalyst. The respective material is impregnated onto silica, fluidized and heated in the presence of oxygen to about 400° C.-860° C., converting chromium from the +3 oxidation state to the +6 oxidation state. A second chromium catalyst used for high density polyethylene applications consists of silylchromate (bis-triphenylsilyl chromate) absorbed on dehydrated silica and subsequently reduced with diethylaluminum ethoxide (DEALE). The resulting polyethylenes produced by each of these catalysts are different in some important properties. Chromium oxide-on-silica catalysts have good productivity (g PE/g catalyst), also measured by activity (g PE/g catalyst-hr) but produce polyethylenes with molecular weight distributions lower than that desired. Silylchromate-based catalysts produce polyethylenes with desirable molecular weight characteristics (broader molecular weight distribution with a high molecular weight shoulder on molecular weight distribution curve, indicative of two distinct molecular weight populations).
Monoi, in Japanese Patent 2002020412 discloses contain the use of inorganic oxide-supported Cr+6-containing solid components (A) prepared by sintering under nonreducing conditions, dialkylaluminum functional group-containing alkoxides (B), and trialkylaluminum (C). The resulting ethylene polymers are said to possess good environmental stress crack resistance and good blow molding creep resistance. U.S. Application 2002/0042482 discloses a method of ethylene polymerization in co-presence of hydrogen using a trialkylaluminum compound-carried chromium catalyst (A), wherein the chromium catalyst is obtained by calcination-activating a Cr compound carried on an inorganic oxide carrier in a non-reducing atmospheric to convert Cr atoms into the hexavalent state and then treating A with a trialkylaluminum compound in an inert hydrocarbon solvent and removing the solvent in a short time.
Hasebe et al. Japanese Patent 2001294612 discloses catalysts containing inorganic oxide-supported Cr compounds calcined at 300° C.-1100° C. in a nonreducing atmosphere, R3-nAlLn (R=C1-12 alkyl; L=C1-8 alkoxy, phenoxy; 0<n<1), and Lewis base organic compounds. The catalysts are said to produce polyolefins with high molecular weight and narrow molecular weight distribution.
Hasebe et al., in Japanese Patent 2001198811 discloses polymerization of olefins using catalysts containing Cr oxides (supported on fire resistant compounds and activated by heating under nonreductive conditions) and R3-nAlLn (R=C1-6 alkyl; L=C1-8 alkoxy, phenoxy; n>0.5 but<1). Ethylene is polymerized in the presence of SiO2-supported CrO3 and a reaction product of a 0.9:1 MeOH-Et3Al mixture to give a polymer with melt index 0.18 g/10 min at 190° under 2.16-kg load and 1-hexene content 1.6 mg/g-polymer.
Da, et al, in Chinese Patent 1214344 teaches a supported chromium-based catalyst for gas-phase polymerization of ethylene prepared by impregnating an inorganic oxide support having hydroxyl group on the surface with an inorganic chromium compound aqueous solution; drying in air; activating the particles in oxygen; and reducing the activated catalyst intermediate with an organic aluminum compound. 10 g commercial silica gel was mixed with 0.05 mol/L CrO3 aqueous solution, dried at 80-120° C. for 12 h, baked at 200° C. for 2 h and 600° C. for 4 h, reduced with 25% hexane solution of diethylethoxyaluminum to give powder catalyst with Cr content 0.25% and Al/Cr ratio of 3.
Durand, et al, U.S. Pat. No. 5,075,395, teaches a process for elimination of the induction period in the polymerization of ethylene by bringing ethylene in contact under fluidized-bed polymerization conditions and/or stirred mechanically, with a charge powder in the presence of a catalyst comprising a chromium oxide compound associated with a granular support and activated by thermal treatment, this catalyst being used in the form of a prepolymer. The Durand process is characterized in that the charge powder employed is previously subjected to a treatment by contacting the said charge powder with an organoaluminium compound, in such a way that the polymerization starts up immediately after the contacting of the ethylene with the charge powder in the presence of the prepolymer.
Unique to chromium-based catalysis generally, molecular weights increase as residence time of the reaction increases. Thus, increasing residence time allows one to achieve higher molecular weight polymers from chromium oxide-based catalysts. However, an increase in reactor residence time represents a decrease in reactor throughput and an increase in production costs. Lowering residence times may lead to better economics but for any particular chromium-based catalyst, also lead to lower polymer molecular weights. To help preserve higher molecular weights, one may decrease reactor temperature, but this results in reduced heat transfer and lower production rates. Better control of the characteristics of the resulting polyethylene, while simultaneously preserving or improving productivity is desired in chromium-based catalyst systems. It is desirable to preserve desirable molecular weights and catalyst activities with decreased residence times. While the prior art contains these and other examples of the use of Phillips-type catalysts and an organoaluminum compound in combination, there has not yet been disclosed a method for obtaining a polyethylene having moderate-to-high molecular weight using a catalyst system having good productivity and in which the molecular weight and molecular weight distribution may be tuned and side chain branching may be controlled. Additionally, the prior art is devoid of any teaching of the use of the in-situ addition of aluminum alkyls (directly to the reactor) to comprehensively address the problems encountered with higher reactor throughput and shorter residence time (polymer molecular weight, molecular weight distribution and catalyst productivity). The present invention addresses a number of the shortcomings of chromium-based ethylene polymerization not previously addressed in the prior art.